1. Field of the Invention
This invention relates generally to the field of monitoring of isothiazolones in aqueous systems, such as manufacturing process waters and cooling towers with concentrations of about 0.5 to 50 mg/l and more specifically to a method for the rapid measurement of isothiazolones.
2. Description of the Related Art
Isothiazolones, as defined herein, refer to substituted and unsubstituted 3-isothiazolones and mixtures having the structural formula:
where R is hydrogen, an unsubstituted or substituted alkyl group, an unsubstituted or substituted cycloalkyl group, an unsubstituted or substituted aralkyl group, or an unsubstituted or substituted aryl group;
X and Y are independently a hydrogen atom, a halogen atom or a (C.sub.1-C.sub.4) alkyl group or when taken together form a substituted or unsubstituted benzene ring to give a compound of the formula:
where Z is a (C.sub.1-C.sub.4) alkyl group, a (C.sub.1-C.sub.4) alkoxyl group, a cyano group, a nitrogen group, or a halogen group; and n is a integer of from zero to two.
Some of the isothiazolones present in aqueous systems may be in the form of complexed divalent salts such as magnesium or calcium.
Isothiazolones, marketed by Rohm and Haas Company and ICI, under the trademarks Kathon®, and Proxel®, respectively, are antibacterial agents or biocides which are widely used in a variety of aqueous and non-aqueous systems. For example, isothiazolones are useful as microbicides in metal working fluids, cooling tower waters, paper making waters, and textile manufacturing process waters. In addition, isothiazolones are added to a wide variety of manufactured solutions such as latex, and cosmetics and to consumer products to control the growth of microorganisms.
Attempts have been made to develop rapid, reliable and sensitive methods for determining the concentration of isothiazolones. These methods generally used sophisticated and expensive gas chromatographic, liquid chromatographic (HPLC) techniques or were colorimetric analyses directed toward field monitoring.
The HPLC methods, although accurate, are expensive to perform because they require highly trained personnel and sophisticated equipment. In addition, the analyses can take several hours to perform, especially if a column on the HPLC unit has not been prepared.
Prior field monitoring colorimetric methods have been found to be less than satisfactory because of the susceptibility of the methods to positive and negative interferences caused by additives to the aqueous systems, ionic impurities commonly found in aqueous systems, and turbidity caused by both soluble and insoluble compounds. Various additives are typically added to recirculating cooling tower water to prevent or inhibit the precipitation of hardness ions, to disperse scale, and to combat corrosion. For example, polyacrylates, phosphates, phosphonates, iron, zinc, tin and other metals are commonly found in cooling tower water as well as suspended particulate materials such as clay and silt.
However, the use of isothiazolones in some manufacturing process waters has brought new challenges in monitoring as isothiazolone concentrations must be monitored more frequently, such as hourly, by personnel having little lab experience. Further, results of monitoring must be known quickly, sometimes within minutes. In this manner, adjustments to process systems can be made immediately to prevent adverse impact to product quality.
As such, a method is needed which eliminated the eliminates potential interferences due to impurities and turbidity, can be performed quickly, can be performed by personnel of different skill levels with minimal training, and uses equipment which is readily available, reliable, and requires a minimum of maintenance.
U.S. Pat. Nos. 3,975,155 and 4,110,378 are directed to prior colorimetric determinations methods of isothiazolones in aqueous and non-aqueous systems. Partial elimination of interferences was achieved by adsorption of the isothiazolones onto an adsorbent resin, desorption of the isothiazolones, and then creation of a calorimetric reaction. Pretreatment of samples by centrifuging was required if very turbid or containing many solids, and other chemicals present could react and produce a colorimetric reaction so these chemicals had to be neutralized first. Identifying all these potential reactionary compounds could prove difficult with manufacturing waters containing many compounds.
U.S. Pat. No. 4,652,530 discloses a calorimetric determination method that also uses adsorption and desorption of the isothiazolones but onto a non-polar adsorbent. The desorbed isothiazolones are then reacted with ferric chloride and potassium ferricyanide to produce a blue color. This method also attempts to eliminate interferences through adsorption and desorption.
There were limitations with this method in that the pH of the sample for adsorption was critical. A sample pH of 10 was required as it was reported that above pH 10 the isothiazolone ring may be cleaved thereby rendering the test inaccurate and that a pH below 10 would adversely affect adsorption. In addition, this method required the use of cyanide compounds which in facilities making Food and Drug Administration (FDA) approved products would be undesirable to even have on the premises.
U.S. Pat. No. 5,094,957 discloses another calorimetric determination method that uses aromatic thiol salts added to a sample to form a colored solution. No pretreatment of the sample is performed.
Limitations with this method were that the color formed in this reaction was unstable and degraded over time. The proposed method required waiting two hours before reading the color absorbance. A two hour wait may be unacceptable for many manufacturing water systems. The method was temperature dependent and thus there was another parameter to be controlled. The method also prevented groups of samples from being measured at the sample time unless the addition of the thiol salts to each sample was timed and tracked. In addition, as with all calorimetric tests, if the sample water was highly turbid, accurate calorimetric readings were difficult to obtain.
As discussed above, current methods for the measurement of isothiazolones use:                1) expensive laboratory equipment such as HPLC, that also require highly trained personnel to both operate and maintain the equipment, and is costly and time consuming if performed by an outside laboratory;        2) calorimetric analyses that do not work when the water is turbid and contains numerous other compounds;        3) methods that take several hours to perform;        4) calorimetric methods in which the color changes over time; and        5) chemistry methods requiring the handling of hazardous materials such as cyanides, that are undesirable at many facilities to even have on the premises.        